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Matter, Dalton’s atomic theory and atom/molecule/element/compound concepts: NEET MCQs with answers

54 questions on this topic of Some Basic Concepts in Chemistry (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. Consider the following microscopic representations of four gaseous samples: Sample 1: Individual helium atoms Sample 2: Nitrogen dioxide molecules Sample 3: Diatomic fluorine molecules Sample 4: A combination of oxygen gas and argon gas Which sample represents a hetero-atomic molecule as its sole constituent?

  1. Sample 2
  2. Sample 1
  3. Sample 3
  4. Sample 4
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Answer: (A) Sample 2

Hetero-atomic molecules are composed of atoms of two or more different elements bonded together. Nitrogen dioxide (NO₂) consists of nitrogen and oxygen atoms, making it hetero-atomic. Helium is monoatomic, F₂ is homo-diatomic, and the mixture of O₂ and Ar is a mixture.

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2. Match List-I (Postulates/concepts related to Dalton's atomic theory) with List-II (Chemical laws or phenomena): List-I A. Atoms of different elements combine in simple whole-number ratios to form compounds B. Atoms can neither be created nor destroyed in a chemical reaction C. Discovery of isotopes of an element D. Discovery of isobars List-II I. Direct contradiction of the postulate that atoms of different elements have different masses II. Law of Definite Proportions III. Direct contradiction of the postulate that all atoms of a given element have identical mass IV. Law of Conservation of Mass Choose the correct answer from the options given below:

  1. A-II, B-IV, C-I, D-III
  2. A-IV, B-II, C-I, D-III
  3. A-II, B-IV, C-III, D-I
  4. A-IV, B-II, C-III, D-I
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Answer: (C) A-II, B-IV, C-III, D-I

Postulate A explains the Law of Definite Proportions (fixed whole-number combination). Postulate B accounts for the Law of Conservation of Mass. The existence of isotopes contradicts the idea that all atoms of an element are identical in mass (C-III), while isobars (different elements with the same mass number) contradict the postulate that atoms of different elements always differ in mass (D-I).

hard

3. Which of the following modern scientific observations serves as an exception to Dalton's original postulate that 'atoms of different elements possess completely different masses'?

  1. The existence of isotopes such as ³⁵Cl and ³⁷Cl
  2. The existence of isobars such as ⁴⁰Ar and ⁴⁰Ca
  3. The existence of allotropes such as diamond and graphite
  4. The formation of non-stoichiometric hydrides
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Answer: (B) The existence of isobars such as ⁴⁰Ar and ⁴⁰Ca

Dalton postulated that atoms of different elements have different masses. Isobars are atoms of different chemical elements that have different atomic numbers but identical mass numbers (e.g., ⁴⁰₁₈Ar and ⁴⁰₂₀Ca), directly contradicting Dalton's claim.

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4. Two gaseous samples, X and Y, are analyzed. Sample X can be separated into simpler substances only by a chemical reaction, while Sample Y can be separated into its constituents by fractional distillation. Samples X and Y are best classified respectively as:

  1. A homogeneous mixture and a compound
  2. An element and a heterogeneous mixture
  3. A compound and a homogeneous mixture
  4. A compound and an element
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Answer: (C) A compound and a homogeneous mixture

Substances that cannot be separated by physical techniques but require chemical reactions to yield constituents are chemical compounds. Substances that can be separated by physical methods like fractional distillation without chemical breaking of bonds are mixtures.

easy

5. Which of the following statements is chemically correct regarding pure substances?

  1. The constituents of a compound retain their individual chemical properties.
  2. An element can be broken down into simpler substances by ordinary chemical reactions.
  3. A pure substance has a fixed composition throughout and distinct characteristic properties.
  4. Compounds can be separated into their constituent elements by physical methods such as filtration.
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Answer: (C) A pure substance has a fixed composition throughout and distinct characteristic properties.

A pure substance consists of only one type of particle and possesses a definite, fixed composition with sharp, characteristic chemical and physical properties throughout. Constituents of compounds do not retain their individual properties and cannot be separated by physical methods.

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6. Which of the following pairs of phenomena both directly contradict Dalton's postulate that 'atoms of the same element are identical in all respects, including mass'?

  1. Allotropy and radioactivity
  2. Isotopy and existence of isobars
  3. Fractional atomic masses of chlorine and existence of ¹H, ²H, ³H
  4. Nuclear fission and law of multiple proportions
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Answer: (C) Fractional atomic masses of chlorine and existence of ¹H, ²H, ³H

Dalton stated that all atoms of a given element have identical mass and other properties. The discovery of isotopes (such as protium, deuterium, and tritium) and the resulting fractional average atomic masses (e.g., chlorine having atomic mass ≈ 35.5 u due to ³⁵Cl and ³⁷Cl) directly contradict this postulate.

easy

7. Which of the following statements is NOT a postulate of Dalton's atomic theory?

  1. Matter consists of indivisible atoms.
  2. Atoms of different elements combine in a fixed, whole-number ratio to form compounds.
  3. Equal volumes of all gases under identical conditions of temperature and pressure contain equal numbers of molecules.
  4. Chemical reactions involve reorganization of atoms; these are neither created nor destroyed in a chemical reaction.
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Answer: (C) Equal volumes of all gases under identical conditions of temperature and pressure contain equal numbers of molecules.

Dalton's atomic theory proposed that matter consists of indivisible atoms, compounds form in fixed whole-number ratios, and chemical reactions involve reorganization of atoms. The statement regarding equal volumes of gases containing equal numbers of molecules is Avogadro's Law, which Dalton actually failed to explain correctly.

easy

8. Which of the following is a compound?

  1. Oxygen
  2. Water
  3. Air
  4. Brass
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Answer: (B) Water

Water has hydrogen and oxygen combined in a fixed ratio and cannot be separated physically. Air and brass are mixtures; oxygen is an element.

NCERT Class 11

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9. Consider the following statements regarding elements and compounds: (a) An element consists of only one type of particle, which may be atoms or molecules. (b) The constituents of a compound cannot be separated into simpler substances by physical methods. (c) Water contains hydrogen and oxygen in a 2:1 ratio by mass. (d) The properties of a compound are completely different from those of its constituent elements. The correct statements are:

  1. (a), (b) and (d) only
  2. (a), (c) and (d) only
  3. (b), (c) and (d) only
  4. (a) and (b) only
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Answer: (A) (a), (b) and (d) only

Statements (a), (b), and (d) are correct. Statement (c) is incorrect because water contains hydrogen and oxygen in a 2:1 ratio by number of atoms (or volume of gases upon electrolysis), but by mass, the ratio is 2× 1 : 16 = 1:8.

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10. Given below are two statements, one is labelled as Assertion (A) and the other is labelled as Reason (R): Assertion (A): Dalton's atomic theory was unable to explain Gay-Lussac’s Law of Gaseous Volumes. Reason (R): Dalton did not recognize the distinction between ultimate particles of elements that take part in reactions (atoms) and ultimate particles that exist independently (molecules). In the light of the above statements, choose the correct answer:

  1. Both (A) and (R) are true and (R) is the correct explanation of (A)
  2. Both (A) and (R) are true but (R) is not the correct explanation of (A)
  3. (A) is true but (R) is false
  4. (A) is false but (R) is true
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Answer: (A) Both (A) and (R) are true and (R) is the correct explanation of (A)

Dalton believed that atoms of the same element do not combine with each other to form diatomic or polyatomic molecules, and thus could not distinguish between an atom and a molecule. This limitation prevented his theory from explaining Gay-Lussac's law of combining volumes, which was later resolved by Avogadro.

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11. Which of the following groups contains only pure chemical substances?

  1. Brass, 22-carat gold, graphite
  2. Milk, petroleum, diamond
  3. Air, sea water, distilled water
  4. Glucose, dry ice, copper wire
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Answer: (D) Glucose, dry ice, copper wire

Glucose (compound, C₆H₁₂O₆), dry ice (compound, solid CO₂), and copper wire (element, Cu) have definite, fixed chemical compositions and are pure substances. Brass, 22-carat gold, air, milk, and petroleum are mixtures.

easy

12. Which of the following species is correctly identified as a hetero-triatomic molecule?

  1. O₃
  2. NH₃
  3. P₄
  4. H₂O
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Answer: (D) H₂O

A hetero-triatomic molecule consists of three atoms belonging to more than one chemical element. H₂O contains two hydrogen atoms and one oxygen atom (total 3 atoms of 2 different elements), whereas O₃ is homo-triatomic and NH₃ is hetero-tetratomic.

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